1. Field of the Invention
The present invention relates to a fully electronic telephone for use in a telephone network in which a DC power signal is supplied to the telephone from a central office over a pair of conductors and AC communication signals are coupled between the telephone and the central office over the same pair of conductors.
2. Description of the Prior Art
The operating environment for a telephone is often not gentle. Telephone equipment, for example, must be capable of withstanding lightning surges of thousands of volts. Furthermore, a telephone must be capable of operating under many diverse electrical conditions. For example, the audio transmit and receive network of a telephone must be capable of operating with substantially less than 4 volts power supply. For example as little as 1.8 volts power supply is typically available when the telephone is located a long distance away from a central office although higher voltages are available at locations adjacent to a central office. Telephone impedance, whether acting as a source or a load, must be capable of changing in what is an awkward manner in comparison to characteristic electronic circuit operation. Specifically, the A.C. impedance of a telephone must be high and variable with line current, and D.C. resistance must be low.
In the past, all of these varying requirements have been traditionally satisfied by non-electronic telephones which employ hybrid transformer coupling of the transmitter and receiver sections to the telephone lines. There are, however, distinct advantages in employing electronic circuits in telephones. For example, many telephone functions now considered to be extra features could be easily installed in an electronic telephone, and many features not previously available in standard telephones could be offered to attract customers such as digital display, repertory or last number dialing, calculators, clocks, printers, and more. There are also potential manufacturing economies to be realized due to reduced parts count and simpler assembly.
To meet the requirements of the present day telephone networks, an electronic telephone must be capable of operating at extremely low D.C. voltages, and must provide the necessary high A.C. impedance required to match the telephone to the network. An electronic telephone would also have to include a two-way transmission amplifier which derived its D.C. source voltage over the same lines which contain A.C. communications signals. Accordingly, an electronic telephone must exhibit a low D.C. resistance at the same time it exhibits a high A.C. impedance. Consideration must also be given to equalizing the operation of the electronic telephone so that the telephone produces a reasonably uniform operation both near and far away from the central telephone office. Finally, it is also necessary that the transmit and receive sections of an electronic telephone be arranged in such a manner that while meeting all of the above-mentioned criteria, they nevertheless provide for sidetone balance which prevents overloading of the receiver section by operation of the transmitter section.
The transmit and receive sections in standard telephones do not incorporate solid-state techniques. Even in the most advanced telephones, in which solid-state amplifiers may ocsasionally be employed, either the transmit and receive sections are coupled to the communication lines through a hybrid transformer or the transmit and receive sections are not capable of DC voltage operation substantially below 4 volts DC, for example on the order of only 1.8 volts. Indirect coupling represents a substantial cost in the manufacture of the telephone and precludes the utilization of pure integrated circuit manufacturing techniques. Failure to operate satisfactorily at low DC voltage precludes commercial acceptance in the United States.
It is, therefore, an object of the present invention to provide a reliable electronic telephone which meets the demanding design requirements necessary for successful operation in existing telephone networks.
Another object of the present invention is to provide an electronic telephone which includes a two-terminal, two-way, solid-state transmission amplifier having low D.C. resistance and high A.C. impedance.
A further object of the present invention is to provide an electronic telephone which includes a two terminal, two-way, solid-state amplifier having low D.C. resistance and high A.C. impedance and which further includes circuitry for equalizing the operation of the amplifier at various distances from a central telephone office.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.